Tools with three different pin profiles: tapered cylindrical, tapered square, and square, were used to friction stir weld 6 mm thick 6063-T4 aluminum alloy plates. Cross-sectional macro-graphs, tensile tests, and fractographs were used to characterize weld quality as a function of tool pin profile and process parameters. Microhardness contours in the weld cross-section and tensile tests on specimen were cut at 0,45, and 90 degrees to the weld line were used to characterize the mechanical response of the weld zone. For the range of process parameters considered, welds obtained using the square tool contained no defects whereas welds made using tapered tools contained large tunnel defects, particularly at higher welding speeds. Further, while the tunnel defects from tapered square tool pin were oriented in the plane of the sheet, the defects from tapered cylindrical tool pin were oriented along the sheet thickness, making them more detrimental. The tunnel defects resulted in a mechanical instability much before macroscopic necking set in.
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